From what you described as the issue with those two fryers, I'd be searching for a ventilation issue within them. Sounds like the flue vents at the rear of the fryers may have an obstruction or there's soot buildup inside the heat exchanger tubes.

If the burners can't properly vent through the flue, then flame exhaust gases will then try to vent back out of the front and, with those vent gases being HOT (hot air RISES, ya know), will tend to build up inside that void under the front shelf of the fryer.

With oxygen-deficient vent gases building THERE, you'll get an odd mixture of of the vent gases and incoming combustion air that forces the burner's upper flames to waft around in search of any oxygen it can grab a hold of. Hence, as you'd described, a lazy-looking flame.

I'll bet it's far worse when the door is closed and you can't see it, since even LESS incoming combustion air is available then.

Propane tends to cause soot more readily than natural gas, so I'd be looking for that. So...turn the fryers off and get a flashlight. Open the fryer's doors and look in past the burners and into each of the heat changer tubes. If you can see any black, "pillowy" formations in there, then that's heavy buildup of soot.

It needs to be removed, but don't touch it! It's r-e-a-l-l-y hard to wash it off your hands. I recommend getting a shop-vac with one of those pricey cartridge filters (Craftsman, DeWalt or others). DO plan to throw that thing away (the filter) right after you're done with the fryers.

With that in mind, remove the burners and have at it. After you're done, put them back together and the fryers should be able to breath again.